The present invention relates to an e-beam deposition method and apparatus for oxide films and, more particularly, to the use of a metallic target and oxygen ambient to produce high purity oxide films.
Certain oxide films, for example, tantalum oxide, are often used as a dielectric coating for laser facets and other optic devices. Compared to various other coatings (such as silicon dioxide or silicon nitride), tantalum oxide has been found to provide superior optical and electrical qualities, as well as improved environmental stability when compared with other oxide materials. The conventional prior art practice of depositing tantalum oxide films utilizes tantalum oxide (Ta2O5) target material in an e-beam deposition chamber, the tantalum oxide target material being provided in granular, chunk or slug form. The target material is placed in a crucible and heated by electron impact to a temperature sufficient to produce a significant Ta2O5 vapor pressure.
E-beam deposition of oxide materials is typically troublesome for a number of reasons. First, the commercially available target oxide materials are not of high purity.
Additionally, oxides by their very nature are poor conductors of electrical current. In e-beam processing, this poor conduction quality produces surface charging of the target source material, making it extremely difficult to control the distribution of the e-beam current striking the target. As a result, the oxide target heats unevenly, producing irregular evaporation rates, spatially non-uniform flux distributions, and xe2x80x9ctunnelingxe2x80x9d of the beam into the target material. All of these problems result in inefficient use of the oxide target material, as well as unpredictable flux distributions. As a result, the deposited oxide films are of relatively low purity and are often distributed in a non-uniform manner across the facet surface.
A need remains in the prior art, therefore, for an improved process of depositing oxide films, such as tantalum oxide, on exposed facet surfaces of optical devices.
The need remaining in the prior art is addressed by the present invention, which relates to an e-beam deposition method and apparatus for oxide films and, more particularly, to the use of a metallic target and oxygen ambient to produce high purity oxide films. In accordance with the present invention, a metallic target, such as tantalum or tungsten, is used as the source for the e-beam deposition process. The metallic target material is then heated and a gas source is used to supply a stream of oxygen over the surface of the heated target. The oxygen stream functions to oxidize a minimal surface depth of the metallic target, where the oxide layer is thick enough to allow for the vaporized oxide to escape the target upon application of an e-beam current. Since the vapor pressure of the oxide is approximately four orders of magnitude greater than the metal, the oxide will preferentially vaporize in the presence of the e-beam current. An advantage of the apparatus and method of the present invention is that commercially supplied metallic tantalum (or tungsten) is generally higher in purity than the prior art tantalum (tungsten) oxide targets, resulting in the deposition of a higher quality film. Further, since a metallic target, by its nature, is more conductive than an oxide target, the metallic target of the present invention will exhibit a significantly lower surface charge than the insulative prior art oxide target, resulting in reduced deflection of the incoming e-beam and improved uniformity of the beam current density across the surface of the target. Additionally, the improved thermal conductivity of the metallic target, when compared with the oxide target, allows for the heat to be spread more uniformly over the target surface, further improving the uniformity of the surface oxide generation.
Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.